1. Field of the Invention
The instant invention relates to a closed loop hydraulic control system which maintains an average pressure in a fluid actuator and permits the pressure on each side of the actuator to change in response to load changes.
2. Description of the Prior Art
A fluid actuator, such as a piston and cylinder, is commonly used to operate or position such devices as a control surface on an aircraft. External forces on such surfaces can produce "flutter" or dangerous high frequency oscillations. An important factor in controlling flutter is to maintain a high amount of "stiffness," or force on the actuator piston to resist movement. An actuator will have maximum stiffness when fluid pressure in both chambers in the actuator can respond to a wide range of load changes.
A single pump driven by a prime mover has been used to control a balanced actuator, i.e. one in which equal volumes of fluid are displaced on either side of the piston during operation. However, a single pump system has one side of the actuator connected to suction pressure. A disadvantage of this system is that fluid pressure in the side of the actuator connected to suction pressure can only respond within a narrow range of pressures. Consequently, actuator stiffness is less than what it would be with both sides responding to a wide range of load changes.
Frequently an unbalanced actuator, i.e. one in which different volumes of fluid are displaced on either side of the piston during operation, is used in a control system. To use an unbalanced actuator with a servo pump, a shuttle valve, which connects one side of the actuator to a separate fluid supply while the other side is connected to tank, may be used to provide the differential flow required. A disadvantage of this arrangement is that the side of the actuator which is connected to tank can only respond to a narrow range of load changes. Thus, again, there is a loss of stiffness in the system.
It is desirable to have a fluid actuator in a closed loop system in which both sides of the actuator piston can respond to a wide range of load changes by permitting the pressure on both sides of the piston to fluctuate in response to external loads applied to a device operated by the actuator, but limiting the sum of the pressures to a predetermined level.